Gas diffusers designed to produce fine gas bubbles in a body of liquid through the process of bubble shearing have been known for some time. U.S. Pat. No. 3,650,513 issued to Werner on Mar. 21, 1972 provides one of the most recent examples of such bubble shearing apparatus, in the form of a rotating disk with porous surfaces on both the top and the bottom of the disk.
The rotating gas diffuser disclosed in the Werner patent just referred to has several structural features that seriously limit the overall size of the apparatus, as well as the pressure at which the gas to be introduced into the body of liquid being treated can be fed into the gas diffuser. Both of these limitations greatly restrict the usefulness of a rotating gas diffuser in bubble shearing. A large diameter of, say, 4 to 10 feet and a gas pressure in the range of 10 to 30 p.s.i. above the hydrostatic pressure at the depth at which the bubble shearing diffuser is to be operated are both very desirable. The larger the diameter of the rotating gas diffuser, the greater the potential area of the porous surfaces on the rotating disk diffuser, and thus the greater the likelihood that a desirably high gas flow rate, such as for example 1500 lbs. per day, can be achieved. Operating pressures of 10 to 30 p.s.i.g. are required to produce the desired gas flow rate through typical porous materials, such as ceramic tile, that are suitable for incorporation in a rotating gas diffuser.
If an open, unobstructed chamber such as the chamber disclosed in the Werner patent is constructed, for example, with a 7 foot diameter, both the upper wall and the lower wall of the chamber would be subjected to a total force of about 110,000 pounds if the apparatus is operated at a gas pressure of 20 p.s.i. above the surrounding hydrostatic pressure. Even a disk constructed with steel plates 3/4 inch thick on both the top and bottom of the chamber would fail under this pressure if there were no internal supports. If the structure were strengthened to withstand the indicated gas pressure, the resulting weight of the rotating diffuser would be so great that the critical speed of the shaft (i.e., the speed at which the first harmonic frequency of the rotating body would result in undesirable vibration, which speed is a function of the mass suspended from the shaft) would be much lower than the speed of rotation at which such a bubble shearing apparatus should desirably be operated. To avoid this result, a rotating gas diffuser of the indicated design would have to be constructed of a much smaller size, or operated at a much lower gas pressure, than is desirable.
Another disadvantage of an open, unobstructed gas chamber such as the chamber disclosed in the Werner patent stems for the large total volume of the chamber. When any rotating gas diffuser loses internal pressure for some reason, the gas chamber will unavoidably flood. Whatever water comes into the chamber during such flooding must be included as part of the mass of the rotating disk when the critical speed of the rotating diffuser shaft is determined. It is true that the major portion of the water will probably be blown out of the gas chamber when the chamber is again raised to its operating pressure, but in the meantime extensive damage could occur if the diffuser was rotated and the critical speed of the shaft was reached while the temporary heavy flooding condition continued. This problem is greatest with an internal gas chamber that extends throughout the full volume of the rotating gas diffuser, as in the Werner prior art apparatus.
Still another disadvantage of the Werner diffuser disclosed in his FIGS. 1-6 arises from the fact that the gas plenum extends radially outward under the non-porous annular periphery of the disk there shown. If the gas plenum floods because of loss of internal pressure as just described, when rotation of the disk is resumed a certain quantity of the water cannot be blown out through the porous part of the walls of the gas plenum because it is caused by centrifugal force to move into the space beneath the non-porous, tapered perimeter of the disk, and is trapped there. The increased weight of the disk due to the presence of this permanently trapped water will therefore continue to affect the critical speed of the diffuser shaft.
All these disadvantages found in the prior art are eliminated by the rotating gas diffuser of the present invention.